1. Field of the Invention
This particular invention generally pertains to an apparatus for drilling and casing earth formations and, more particularly, to a novel and improved rotary down-the-hole percussive drill apparatus.
2. Description of the Prior Art
In many particular drilling applications such as water wells, injection and disposal wells, coring, and exploration, it is desirable and often essential that well casing be installed while drilling or upon completion of the drilling process. The casing is installed to reduce or to prevent the risk of lost drill tools or loss of hole, sample contamination, ground water pollution and/or seepage, or to facilitate pumps and other hardware.
At present casing of wells is performed either by the "drill and drive" technique whereby the casing is mechanically driven through the formation before drilling, during drilling or following the drilling bit or by the installation of the casing into a completed borehole of larger diameter. Drilling in loose and unconsolidated materials and in highly fractured ravelly earth formations is often complicated by hole erosion, hole caving, the presence of hard boulders, or large volumes of ground water; and, consequently the hole is difficult or impossible to case. As a result the hole has to be terminated prematurely, redrilled, or a new hole drilled.
For example, in the field of drilling, it has been rather standard operating practice to drill injection wells, which may vary in depth, on top of or into production leach dumps to introduce a leaching solution for well-known purposes. Heretofore, such well drilling procedure has been accomplished through use of cable tool rigs or churm drills which suffered from certain disadvantages in that they were relatively slow and rather expensive in operation.
One approach which was taken to alleviate the aforenoted disadvantages, so as to enable bringing the aforenoted dumps into production sooner, as well as reduce the costs attendant with drilling, employed the use of a truck mounted rotary drill which was equipped with a conventional down-the-hole hammer drill. Whenever, however, such a technique was utilized for drilling and casing wells in situations wherein such factors as ground water and a relatively high percentage of very coarse and unconsolidated material were present, completion of wells was complicated. Although drilling proved to be relatively fast and easy, by reason of the down-the-hole hammer drill, it was somewhat difficult and, in certain circumstances impossible to install casing since the drilled holes would cave as the drill string was pulled out of the hole. As a consequence thereof, casing or well completion operations in these circumstances were unable to be satisfactorily performed.
Another known approach is generally described in U.S. Pat. No. 3,682,260, wherein there is disclosed a down-the-hole hammer drill with a removable inner drill bit cooperating with an annular drill bit. Such an approach, however, does not provide adequate assurances that the inner drill bit will not be forced outwardly through the open bottom end of the outer drill rod through the repeated hammerings of the hammer drill. Accordingly, if the inner drill were so forced outwardly the drilling apparatus would, of course, fail to perform its intended operation. Additionally, this particular form of construction is somewhat more complicated in that it requires the utilization of an inner drill pipe to rotate the inner drill bit.